1. Field of the Invention
The present invention relates to fuel injectors for diesel engines. In particular, the invention relates to such a fuel injector in which a high pressure gas is mixed with the fuel to obtain improved atomization of the fuel being injected into the engine.
2. Description of Related Art
U.S. Pat. No. 4,167,921 to Steinwart discloses a fuel injector having an air chamber surrounding a fuel injection head, where "air assist" type atomization of the fuel is performed using air that is introduced into the air chamber from the atmosphere or the air induction duct, and from which the atomized fuel is injected through an outlet opening of the air chamber. The injector head is adjustable in relation to the outlet opening in accordance with the load setting of the engine. The cross-section of the outlet opening is variable between that needed for idle running and that needed for full load. However, because the outlet opening of the air chamber is always open and Steinwart introduces air into the air chamber at low pressure downstream of the injection nozzle, precise control of the fuel air cannot be obtained and the system cannot be applied to diesel and other direct injection engine applications.
U.S. Pat. No. 5,241,938 to Takagi et al. discloses a fuel injector having a plurality of cylindrical air passages that allow air to be introduced by suction into a mixing chamber that is open to the engine intake manifold in directions causing the air form to intersect a supply of fuel introduced from a fuel injector into the mixing chamber for atomizing the fuel. As is the case for the Steinwart system, precise control of the fuel air cannot be obtained and the system cannot be applied to diesel and other direct injection engine applications.
However, with the imposing of more stringent diesel engine emissions regulations, high efficiency atomization of the fuel is necessary to produce combustion with the required low levels of emissions. One key measure of spray quality is the Sauter Mean Diameter (SMD) of the spray droplets, with respect to which the smaller the SMD, the better the spray quality with spray droplet sizes in the range of 5-6 micron SMD being sought. Unfortunately, to achieve a spray with droplets of such a size, current liquid fuel injection systems demand extremely high pressures, unique materials and expensive manufacturing techniques.
Thus, a need exists for a fuel injection system which can obtain the desired degree of atomization at lower pressures, so that manufacturing costs can be reduced while still meeting emissions requirements. A type of atomizer which is known to obtain significant improvement in liquid atomization relative to air assist atomizers at lower pressures is an effervescent atomizer. In effervescent atomization, a gas is injected into the flowing liquid stream upstream of a discharge orifice and upon exiting the orifice the gas expands very quickly causing the liquid to break up into small drops. However, fuel injectors for internal combustion engines which operate on the effervescent atomization principle are, as of yet, unknown.